1. Technical Field
This invention relates generally to memory devices, and more particularly, to a memory array which includes a plurality of resistive memory devices.
2. Background Art
FIG. 1 is a schematic representation of a portion of a typical DRAM memory array 100. The array 100 includes a plurality of word lines (two shown at WL0, WL1), and a plurality of bit lines (one shown at BL0). The array 100 includes a large number of similar memory cells (two memory cells MC0, MC1 shown in FIG. 1). The memory cell MC0 includes a transistor T0 having its drain D0 connected to the bit line B0 and a capacitor C0 having first and second plates C0P1, C0P2, plate C0P1 being connected to the source S0 of the transistor T0. The word line WL0 is connected to the gate G0 of the transistor T0. Likewise, memory cell MC1 includes a transistor T1 having its drain D1 connected to the bit line B0 and a capacitor C1 having first and second plates C1P1, C1P2, plate C1P1 being connected to the source S1 of the transistor T1. The word line WL1 is connected to the gate G1 of the transistor T1. The plates C0P2, C1P2 of the respective capacitors C0, C1 are connected, giving rise to the term “common plate” (CP).
It will be understood that the two cells MC0, MC1 shown and described are part of a large number of such memory cells in the array 100.
The data storing mechanism of each memory cell is based upon the presence or absence of electric charge accumulated in the capacitor. The presence or absence of the electric charge in the capacitor can be sensed by means of sense amplifier SA (connected to a bit line BL0), sensing current in the bit line BL0. The programming and erasing of each cell is undertaken in a well known manner.
Recently, an approach has been undertaken where, in effect, each of the capacitors of the array 100 of FIG. 1 is replaced by a metal-insulator-metal (MIM) resistive memory device. Such a device is capable of adopting a low-resistance (“programmed”) state and a high resistance (“erased”) state by application of electrical potentials thereacross, so that upon application of an appropriate electrical potential applied across the device, the level of current therethrough can be sensed, indicating whether the device is in its high or low resistance state (the “read” step).
While resistive memory devices of this type provide significant advantages, the proper programming, erasing and reading of devices in an array of the type described have proven challenging. It is of course understood that the programming, erasing and reading of a selected device in the array must be effective and reliable. The goal of the present approach is to achieve a high level of such effectiveness and reliability.